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John Warren - Thursday Feb 10, 2019 VolatileSalty evaporite Matters interactions with magma, Part 1 of 3: Indications of
hydrated salts? teractive and external to the magma). Both encompass outcomes Introduction that can include a variety of substantial ore deposits (Lebedev Direct and indirect interactions between magma and evaporites and Pinsky, 2017; Warren, 2016; Chapter 16; Morteani et al., at a regional scale are neither well documented, nor well un- 2013). Only in situations where igneous sills and dykes have in- derstood. Mostly, this is because little or no salt remains once truded salt masses, with contacts preserved, can direct effects of the high-temperature igneous-driven interactions have run their magma-salt interaction be documented. Even then, determining course; instead, there is a suite of indirect geochemical and in- the timing of the evaporite igneous interaction can be problemat- dicator-mineral assemblages (Warren, 2016; Morteani et al., ic; one must ask if the chemistry and texture indicate, 1) syn-ig- 2013). Aside from the presence of what can be ambiguous in- neous emplacement, or 2) post-emplacement alteration and dicator mineral suites, some hard-rock geologists with careers deeply circulating groundwater flushing, or 3) a combination. working in igneous and metamorphic terranes may not be well Historically, ignous melt interactions are considered in terms of versed in textures indicative of the former presence of sedimen- the dominant subsurface evaporite phases - halite and anhydrite tary evaporites, nor their varying volatility, nor their meta-evap- - both anhydrous salt minerals. In this article we will considered oritic and meta-igneous siblings. also igneous interactions with hydrous salts, like carnallite, ka- The term pyrometasomatic encompasses some, but not all, of the inite, polyhalite and gypsum; any of which can be locally signif- types of salt-magma interaction and reactions that occur when icant bed constituents in a halite-dominant basin fill (Table 1). evaporites and molten magmas of different types are nearby. Orthomagmatic and paramagmatic evaporite associations are Styles of evaporites interactions with magma are a spectrum, distinct from occurrences of primary igneous/magmatic anhy- with two endmember situations; 1) orthomagmatic (salt-assimi- drites, which precipitate from sulphate-saturated melts. Igneous lative and internal to the magma), and 2) paramagmatic (salt-in- anhydrite forms independently of any sedimentary evaporite Salt Formula Decomposition/Melt point (°C) assimilation, as seen, for example, in anhydrite crystals crystal- lised in trachyandesitic pumice erupted from El Chichón Volca- Halite NaCl 804 anhydrous melt no in 1982, or in dacitic pumices erupted from Mount Pinatubo Gypsum CaSO .2H O 100-150 loss of water 4 2 in 1991 and in acidic lavas in the Yanacocha district of northern Anhydrite CaSO 1460 anhydrous melt 4 Peru (Luhr et al., 2008; Chambefort et al., 2008). These evaporite
Carnallite KMgCl3.6H2O 110-120 loss of water assimilations are also distinct from fumarolic anhydrite, which Sylvite KCl 750-790 anhydrous melt precipitates where groundwaters and sulphur-bearing magmatic fluids interact, as on Usu Volcano, Hokkaido, and many central Bischofite MgCl2.6H2O >118 loss of water 712 anhydrous melt American and Andean volcanoes such as El Laco (Zimbelman et al., 2005). Likewise, they are distinct from the anhydrite precip- Epsomite MgSO4.7H2O >70-80 loss of water itates (white smokers) in and below submarine vents across nu- Kieserite MgSO4.H2O 150-200 loss of water 1120-1150 anhydrous melt merous mid-oceanic ridges (Humphris et al., 1995). See Warren Polyhalite K Ca Mg(SO ) .2H O 230-280 loss of water 2016 (Chapter 16) for more geological detail on these non-evap- 2 2 4 4 2 orite-igneous anhydrite occurrences. Trona Na3H(CO3)2.2H2O >70 loss of water Nahcolite NaHCO 270 anhydrous melt 3 Cooking with salt (thermal decomposition TYPICAL MAGMA/ERUPTION TEMPERATURES of hydrated versus non-hydrated salts) Picritic 1400 -1500 Perhaps the most critical factor controlling the local intensity of magmatic interaction with an evaporitic country rock is whether Basaltic 1100-1200 or not the intruded sedimentary evaporite assemblage, in prox- Rhyolitic 800-900 imity to an igneous heat source, contains abundant hydrated Natrocarbonatite 500-600 salts, such as gypsum, polyhalite or carnallite. Hydrated evap- Table 1. Decomposition and melting points of selected salts and typical orite salts, when interacting with the igneous realm, are highly ranges of magma temperatures (after Warren, 2016). Page 1 www.saltworkconsultants.com
volatile and likely to decompose. They 12 tend to release their water of crystalli- 200° sation at temperatures many hundreds 10 of degrees below the melting points of their anhydrous counterparts (Table 1). 8 250° In contrast, anhydrous salts, such as 6 halite beds intruded by igneous dykes 300° or sills, are much less reactive. At a Time (months) Time 4 350° local scale (measured in metres) with 400° respect to an intrasalt-igneous interac- 2 150° tion, there are a number of document- ed thermally-driven alteration styles, 0 1 2 3 4 5 typically created by the intrusion of Distance (m) into salt from edge of a 1.8m-wide basalt dyke dolerite dykes and sills into cooler ha- Figure 1. Temperature effects in salt across time (1 year) and distance (m) from the edge of a 1.8m-wide lite, or the outflow of extrusive igneous basalt intrusion into Zechstein halite in the Fulda region, Germany (after Knipping, 1989; Warren, 2016). flows over cooler halite beds - (Knip ping and Herrmann, 1985; Knipping, alteration halo extends up to twice the thickness of the dolerite 1989; Grishina et al, 1992, 1998; Gutsche, 1988; Steinmann et sill above the sill and almost the thickness of the sill below (Fig- al., 1999; Wall et al., 2010). Hot igneous material interacts with ure 1). somewhat cooler anhydrous salt masses, typically halite or an- Four inclusion type associations were found in bedded halite as hydrite, to create narrow but distinct heat and mobile fluid-re- a function of the ratio of the distance of the sample from the lease envelopes(Figure 1), also reflected in the resulting recrys- intrusion contact (d) to the thickness of the intrusion (h), i.e. d/h tallised inclusion-modified salt textures in a heat halo, centred (Figure 2). Chevron structures with aqueous inclusions progres- on the intrusive (Figure 2). sively disappear as d/h decreases; the disappearance of chevrons Based on studies of inclusion chemistry and homogenization occurs at greater distances above than below the intrusive sill. At temperatures in fluid inclusions in bedded halite near intrusives, d/h < 5 above the sill, a low-density CO2 vapour phase appears it seems that the extent of the influence of a dolerite sill or dyke in brine inclusions, at d/h < 2 H2S-bearing liquid-CO2 inclu- in bedded salt is marked by fluid (brine)-inclusion migration. sions appear, sometimes associated with carbonaceous material
This is evidenced by the disappearance of chevron structures and orthorhombic S8, and for d/h < 0.9, CaCl2, CaCl2.KCl and and consequent formation of clear (sparry) recrystallised halite, nCaCl2.n MgCl2 solids occur in association with free water and with a new set of higher-temperature brine inclusions located liquid CO2 inclusions, with H2S, SCO, and Sg. The d/h values at intercrystal or polyhedral intersections. Such a migration en- marking the transitions outlined above are lower below the sills velope is documented in bedded Cambrian halites intruded by than above. The water content of the inclusions progressively end-Permian dolerite dykes in the Tunguska region of Siberia decreases on approaching the sills, whereas their CO2 content (Figure 2; Grishina et al., 1992). There, as a rule of thumb, an and density increase. Carnallite, sylvite and calcium chloride can occur as solid inclusions in the two associations nearest to the sill Inclusion associations resulting from chevron halite alteration and for d/h<2. Carnallite and sylvite inclusion migration/modi cation halite chevron driven by dolerite sill emplacement occur as daughter minerals in brine Unchanged
I II III IV inclusions. The presence of carbon d/h Association I is characterized by carnallite-bearing brine inclusions with a dioxide is taken to indicate fluid cir- 7 CO vapour bubble in clear halite. 2 culation and dissolution/recrystalli- 6 Association II is characterized by various ratios of liquid CO and brine. 2 sation phenomena induced by the ba- 5 Association III is characterized by the following types of solid salt inclusions salt intrusions. The origin of carbon 4 and uid inclusions: dioxide is likely related to carbonate 3 1) Highly concentrated brine with a metastable solid phase CaCl2·6H2O; 2) Brine with several salt solids: CaCl ·nH O, 2MgCl · 12H O; 2 2 2 2 2 dissolution during magmatism (see 3) Liquid CO2 with solid CaCl2·6H2O; 4) Liquid CO + several salt solids: CaCl ·nH O, 2MgCl ·CaCl · 12H O (± brine); 1 2 2 2 2 2 2 Salty Matters, Oct 31, 2016). 5) Liquid CO2 without brine; 6) Solid salt hydrate; h 7) Anhydrous solid salt In some shallow locations, relatively rapid magma emplacement can lead 1 Association IV is characterized by single-phase brine inclusions and by very rare inclusions with a small CO phase in addition to the brine. In 20 out of 23 2 2 to linear breakout trends outlined by samples, the halite is not massive halite, but lls dissolution holes in 3 dolomite beds. Occurrence of these inclusions does not depend on the d/h phreatomagmatic or phreatic explo- ratio. sion craters. Such phreatic explosion craters have been imaged on the Tertiary seafloor horizons in parts Brine CO2 CO2 Chevron Recrystallised Dolerite vapour liquid halite halite sill of the North Sea (Figure 3; Wall et Figure 2. Occurrence of main fluid inclusion associations as a function of the d/h ratio; where d is al., 2010). The dykes were emplaced the distance of the sample from the dolerite sill that has a thickness h (after Grishina et al., 1992). into Paleozoic and Mesozoic sedi-
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where from 500 to 1,170 °C) can cause a near-instantaneous phase change to steam, so forming a phre- atomagmatic deposit. That is, rapid heating results in an intense explo- sion made up of steam, water, ash, rock, and volcanic bombs. During the eruption of Mount St. Helens, Zechstein salt Phreato-magmatic collapse crater on Cretaceous sea oor atop Tertiary dykes penetrating Zechstein salt A. hundreds of steam explosions pre- ceded the1980 Plinian eruption of the volcano core. Many authors ar- Collapse crater Figure → Reflectors onlap the on seafloor gue a less intense geothermal event crater surface Top Cretaceous results in a mud volcano, but there are many other active mud volca- noes worldwide that tie to compac- tional overpressure unrelated to any magma emplacement (Warren et al., Zechstein salt B. 2011). As the published interpre- tation of aligned phreatic breakout structures illustrated in Figure 3 is Figure → based on seismic without well con- trol, the explosion mechanism may be solely phreatic heating or phrea- tomagmatic. Deposits of phreatic eruptions (as C. contrasted with a phreatomagmatic eruption) typically include steam Figure 3. Interpretation and seismic close-ups of the upper area of a variety of disturbed zone showing the main features. (a) Seismic line and interpretation of corresponding disturbed zones, showing the and rock fragments without the in- location of the dykes A, B and C, which correspond to the creation of broad and narrow disturbed clusion of fragments derived from zones and the underlying dyke location. b) Seismic line through a wide disturbed zone with well liquid magma, lava or volcanic ash. developed high amplitude unit (HA). The internal Chalk seismic reflections truncate against the HA The temperature of the phreatic unit. The drape of reflections can be seen above the top Chalk re£ection. (b) Seismic line and inter- pretation through a narrow disturbed zone. fragments can range from cool to incandescent. So if molten magma ments and have a common upper termination in Early Tertiary is present, the resulting explosive sediments. The dykes are part of the British Tertiary volcanic debris deposit is typically classified as a phreatomagmatic erup- province emplaced some 58 Ma. These dykes are characterised tion. These eruptions can create broad, low-relief craters called by a narrow 0.5–2 km wide vertical disturbance of seismic re- maars. In contrast, phreatic explosions lack debris derived from flections that have linear plan view geometry. Negative magnet- molten (igneous) material, but emplacement can be accompanied ic anomalies directly align with the vertical seismic disturbance by carbon dioxide or hydrogen sulfide gas emissions. CO2 can zones and indicate the presence of underlying igneous material. asphyxiate at sufficient concentration; H2S is a broad spectrum Linear coalesced collapse craters are found above the dykes. poison. A 1979 phreatic eruption on the island of Java killed 140 The collapse craters formed above the dyke due to the release people, most of whom were overcome by poisonous gases. Phre- of volatiles at the dyke tip and resulting gaseous expansion and atic eruptions, even if the deposit lacks igneous rock fragments, subsequent volume loss. According to Wall et al. (2010), the are typically classed as a type of volcanic eruptions because a larger craters likely formed due to explosive phreatomagmatic phreatic eruption can force juvenile fluids to the surface. But interaction between magma and pore water. The linearly aligned when a phreatic explosion is related to an igneous feature inter- collapse craters can be considered an Earth analogue to Martian secting an evaporite bed, the resultant textures show a contrast pit chain craters. between heating of anhydrous and hydrous salts A phreatic eruption, also called a phreatic explosion, ultravulca- nian eruption or steam-blast eruption, occurs when magma heats Hydrous salt interactions in Germany ground or surface water and is a separate but related occurrence Textures created by an igneous intrusion into a variably-hydrat- to a phreatomagmatic eruption. A phreatomagmatic deposit typ- ed evaporite succession can be studied in the dyke-and sill-in- ically contains solid inclusions of magmatic (igneous) material, truded halite levels exposed in the walls of potash mines of whereas debris tied to a phreatic deposit does not, but ties to the the Werra-Fulda district of Germany (Figure 4; Steinmann et effects of juvenile and deeply circulated hydrothermal waters. al., 1999; Schofield et al., 2014). There, the Permian Zechstein Extreme temperatures associated with an emplaced magma (any- salt series contains two important potash salt horizons (2-10m thick), which are mined at a depths ≈ 800 m, from within a 400m Page 3 www.saltworkconsultants.com
N Eschwege Mühlhausen HESSEN South Basalt dykes North Shaft Herrigen Neuhof Shaft Hattorf Shaft Wintershall Bad Hersfeld Eisenach Fulda Graben Werra Bad Salzungen 200 Approx. 100m clay Bunter sandstone 30m dolomite K H 0 1 Salt subrosion Z1 Upper Werra rocksalt -200
-400
Fulda Meiningen Depth (m) -600 K Th Rotliegend 1 Basal Zechstein Z Lower Werra rocksalt 1 5 km BAYERN -800 Steinsalz >100m A. 10 km Kalisalze B. Figure 4. Distribution of layered Zechstein potash in the Fulda Embayment, Germany.A) Map view of the extent of Kalisalze. B) Idealised cross section showing K1H (Hessen Seam) and K1Th (Thüringen (after Warren 2016). thick halite host (Figure 4a). In the later Tertiary, basaltic melts up to 300 m. intruded these Zechstein evaporites, but it seems only a few dykes reached the Miocene landsurface. The basaltic melt ties Two potash seams (Seam Hessen and Seam Thüringen) sepa- to regional volcanic activity, some 10 to 25 Ma. Basalts exposed rate the rock salt of the Werra Formation into three distinct units in the halite-dominant portions of the mine walls are typically (Figure 4b). Lower, Middle and Upper Werra rock salt). Seam subvertical dykes, rather than sills. The basaltic intervals inter- Hessen mainly consists of hard salt (kieserite, sylvite, halite sect the salt over zones up to several kilometres wide (Figure and anhydrite). It is overlain by several, potash mineral-bearing 4b). However, correlations of individual dyke swarms, either horizons which show a strong vertical and lateral heterogeneity between different mines, or between surface and subsurface out- and consist of kieserite, sylvite, carnallite, halite and anhydrite. crops is difficult. Internally, three separate units are identified within the potash Seam Hessen (Figure 4). The “Wurmsalz”, a hard salt with up The dykes and sills are phonolitic tephrites, limburgites, basani- to four strongly folded anhydritic clay bands represents the low- tes and olivine nephelinites. The dyke margins adjacent to halite er part of Seam Hessen. The middle part consists of massive, intervals are usually vitrified, forming a microlitic limburgite kieserite-rich hard salt with abundant sylvite lenses (“Flocken- glass along dyke edges in contact with salt (Figure 5; Knipping, salz”). The “Bändersalz”, a banded hard salt which is typically 1989). At the contact on the evaporite side of the glassy rim, intercalated with brownish, halitic layers occurs in the upper there is a cm-wide carapace of high-temperature salts (mostly part of Seam Hessen. Potash Seam Thüringen usually occurs anhydrite and ferroan carbonates). Further out, the effect of the around 50 m below Seam Hessen. Its lower part is dominated by high-temperature envelope is denoted by transitions to clear ha- a well-bedded hard salt with intercalated rock salt. Its upper part lite, with higher temperature fluid inclusions (Knipping 1989). consists of a variety of rock types including carnallite, sylvite All of this metre-scale alteration in a halite interval is an anhy- and hard salt. drous alteration halo; the adjacent salt did not melt (halite has a melting temperature of 804°C), rather halite recrystallised, so In the Fulda region the thermally-driven release of water of migrating entrained brine inclusions from chevrons into polyhe- crystallisation within particular Zechstein salt beds intersecting dral inter-halite crystal spar positions (as also illustrated in Fig- igneous dykes creates thixotropic or subsurface “peperite” tex- ure 2). The dolerite/basalt interior of the basaltic dyke is likewise tures in hydrated carnallitite ore layers, where heated water of altered and salt soaked, with clear, largely inclusion-free halite crystallisation escaped from the hydrated-salt lattice. Dehydra- typically filling vesicles in the basalt (Figure 5b). tion-driven loss of mechanical strength focuses zones of magma entry into particular horizons in the salt mass, wherever hydrated In contrast, the heating of hydrated salt layers (such as carnal- salt layers were intersected (Figure 5b verses 5c). In contrast, lite or kainite as seen in Figure 5b,c), adjacent to a dyke or sill, dyke and sill margins are much sharper and narrower in zones drives off the water of crystallisation (chemical or hydration of contact with anhydrous salt intervals (Figure 5a, d; Schofield thixotropy) at a much lower temperatures than that at which an- et al., 2014). hydrous salts, such as halite or anhydrite, thermally melt (Shof- ield et al., 2014). Accordingly, away from the immediate vicinity of the direct thermal aureole, heated and overpressured dehydration waters From a paleogeographic perspective, the Werra-Fulda Basin is can enter a former Zechstein carnallite halite bed, and drive the situated in a southern embayment of the European Zechstein Ba- creation of extensive soft sediment deformation and peperite sin. It contains cyclic evaporites of the Werra Formation (Z1). In textures in the previous hydrated layer (Figure 5c, d). Miner- the Neuhof area, the evaporites of the Zechstein are underlain by alogically, sylvite and coarse recrystallised halite dominate the siliciclastic rocks of the Permian Rotliegend interval. The higher salt fraction in the peperite intervals/beds. These deformed beds Zechstein-cycles (Z2 – Z7), on top of the Werra Formation, con- formed within a hydrated salt bed and so differ from the con- sist of a siliciclastic succession with intercalated limestone and ventional notion of volcanic peperites indicating water-saturated anhydrite layers (Strauch et al., 2018; Beer and Barnasch, 2018). sediment interactions with very shallow dyke or sill emplace- The Werra Formation is dominated by rock salt with a thickness ments. Page 4 www.saltworkconsultants.com
A. B.
Linear knife-sharp contact with minor alteration <5cm
Isoclinal folds in carnallite-sylvite -halite ore layer, sandwiched between layered halite. This hydrated salt layer is the zone that evolves into a subsurface peperite layer (via 20 cm thixotropic dehydration)
Knife-sharp contact C. opposite halite D.
Dyke expands into sill with peperite textures
Dyke bulges opposite former carnallite- sylvite ore zone
Figure 5. Tertiary basalt dyke intrusions in Zechstein halite in a potash mine in the Fulda region, Germany. A) Steeply inclined dyke in laminated halite (anhydrous) showing knife-sharp contacts. B) Carnallite-sylvite ore zone showing typical isoclinal folds sandwich between layered halite units with alteration driven by the presence of hydrous carnallitite. C) Near vertical dyke showing knife-sharp contacts opposite layered halite and bulges into the level of a carnallitite/sylvite ore zone. D) Close up of bulge zone. (Schofield et al., 2014; Warren, 2016).
Sylvite in these altered zones is a form of dehydrated carnal- ids were mixed with fluids originating from thermally-mobilised lite, not a primary-textured salt. In the Fulda region, such altered crystallisation water in the carnallite as it converted to sylvite. zones and deformed units can extend along former carnallite layers to tens or even a hundred or more metres from the dyke Nearer the basalt dyke, the carnallite is largely transformed into feeder. Ultimately, the deformed potash bed passes laterally out inclusion-poor halite and sylvite, the result of incongruent flush- into the unaltered bed, which can retain abundant inclusion-rich ing of warm saline fluids mobilised from the hydrated carnallite primary chevron halite and carnallite (Figure 5c versus 5d). That crystal lattice as it was heated by dyke emplacement. During is, nearer the basalt dyke, the carnallite is transformed mainly Miocene salt alteration/thermal metamorphism in the Fulda re- into inclusion-poor halite and sylvite, the result of recrystallisa- gion, NaCl-fluids were mixed with fluids originating from ther- tion combined with incongruent flushing of warm saline fluids mally-mobilised crystallisation water in the carnallite, as it con- mobilised from the hydrated carnallite crystal lattice as it was verted to sylvite. This brine mixture altered the basalts during heated and decomposed in response to nearby dyke emplace- post-intrusive cooling, an event which numerical models sug- ment. During such Miocene salt alteration/thermal metamor- gest was quite rapid (Knipping, 1989): a dyke of less than 0.5 phism in the Fulda region, NaCl-rich diagenetic and juvenile flu- m thickness probably cooled to temperatures less than 200°C within 14 days of dyke emplacement. Page 5 www.saltworkconsultants.com
Worldwide, igneous dykes intersecting salt beds tend to widen 2016 and Bastow et al. 2018 for more detailed discussion of to become sills in two zones: 1) along evaporite units within the potash stratigraphy). To attain these hydrated salt levels the the halite mass that contain hydrated salts, such as carnallite or rising dyke swarm had passed relatively passively through the gypsum (Figure 5b, c) and, 2) where rising magma has ponded Lower Rocksalt Formation (Salty Matters, April 29, 2015). Em- and so created laccoliths at the upper or lower halite contact with placement of the magma/dykes into hydrated evaporites below the adjacent nonsalt strata, or against a salt wall (Warren 2016). the vicinity of what is now the Dallol Mound would have mo- The first alteration of the hydrated salt layer is a form of mineral bilised and deformed the hydrated potash salt level, converting alteration and recrystallisation in response to a pulse of released carnallite to sylvite, kainite to bischofite and lesser kieserite, as water/steam as dyke-driven heating forces the dehydration of well as creating widespread cavities filled with rising pressured hydrated salt layers. The second alteration is often folding and volatiles carried by MgCl and KCl brines. Pressurisation likely fluid-like disaggregation of the former, now dehydrated, layer in created a cavernous network filled with volatiles at the level of response to the mechanical strength contrast at a hydrated-non- the Houston Formation and would have aided in driivg local up- hydrated salt-bed contact (Warren, 2016). lift and doming, so forming the four-way dip closure now seen on the exposed and eroding salt beds that make up much of the Surface expression of hydrated bedded salts Dallol Mound surface. interacting with magma in Dallol, Ethiopia Once the hydrothermal cavities stope and breach their way to Local potash ores typify thermal sump depressions in the Dallol surface, the feeder brines cool and precipitate prograde salts and Musley areas (Figure 6, 7) where a similar set of subsurface (typically anhydrous) dominated by halite, sylvite and bischof- destabilisation processes occurred when rising magma reached ite. Such destabilisation has likely focused the emplacement of a the levels of hydrated salts (kainite and carnallite beds) in the basaltic sill at the level of the potash salts, in turn driving the up- Houston Formation of the Danakhil depression fill (see Warren lift of the lake beds above this region, now outlined by the cen- tripetal dips of the Dallol Mound. Mound-related uplift and hydro- thermal activity focused at the level Uplifted and Abandoned of hydrous salts at the level of the A. Mt Dallol sulphur eroded salt- Parsons Houston Formation, stimulated the springs, area lacks lake sediment mine camp formation of natural areas of ground volcanic debris on Dallol ank collapse, sulphurous and acidic springs and fumaroles, along with the creation of water-filled chimneys and doline sags, filling with various hydrothermal salts, in the vicinity of Musley bajada the volcanic mound. B. That is this type of potash in the Dallol Mound region is hydrother- C. mally reworked from the uplifted Hydrothermal karst equivalents of the Houston Forma- 1 km springs in saltat sediments tion. Even today this hydrology is precipitating carnallitite (associated with bischofite and minor kiese- Chain of small rite) in various hydrothermal brine B. linked-spring dolines C. pools atop and around the Dallol Uplifted and eroded Mound, such as the carnallite-dom- lake sediment rim inant Crescent deposit (Figure 6b). These hydrothermal salts owe their Site of 1926 phreatic eruption Dissolution-induced origins to daylighting of pressurised (30m diam,carnallitite pool) topographic low that is fluid systems and cavities. a brine oored collapse doline The last pressurised phreatic explo- sion crater formed in 1926. They Black Mountain were created by the volatile prod- Bischo te Sediment surface at 500 m 100 m ambient lake level ucts of hydrated salt layers (Hous- pool ton Fm) where these salts had come Figure 6. Hydrothermally-influenced dolines in saltflat south of the Dallol volcanic mound. A) Regional into contact with thermal aureoles overview of Dallol volcanic mound and saltflat. B & C detail of potash rich brines occurring in active or actual lithologies of newly em- solution dolines in the Dallol saltflat fed by hydrothermal brines that have dissolved the more soluble placed dykes that had penetrated portions of uplifted potash-entraining evaporite sediments. Feature C is locally known as Boiling Lake the underlying halite section. Vol- (or Geyser Lake or Acid Lake) and is one of the main tourist attractions. Page 6 www.saltworkconsultants.com
canic rock fragments and other igne- “Black Mountain” SW margin of Dallol Mound, an exposed salt dome surface ous debris have yet to make it to the made up of eroded and uplifted halite beds surface in the Dallol Mound region, although active volcanic mounds and flows do cover the saltflat surface tens of kilometres to the south (Erte Alle ) and north. Based on the analo- gy exposed within the Zechstein-host- ed potash mines of the Fulda region of Germany, it is likely that as well as creating at-surface brine pools, this Hydrothermal salt pool area that is rich in bischo te and carnallite hydrothermal dyke-related hydrology locally converts most subsurface car- nallitite to a disturbed sylvinite bed at the level of contact with the Houston Fm. Figure 7. View north across and pond area with carnallite, bischofite and halite precipitates toward the southwest edge of Dallol mound (salt dome) near the metres-high edge of the Black Mountain Implications ridge. Both the main Dallol Mound and Black Mountain are made up of centripetally- dipping and It seems a "one-size-fits-all" model uplifted lake halite beds. does not characterise magmatic in- rons. What makes these hydrous-salt peperites interesting is that teractions with massively bedded evaporites. Instead, there is a it is the igneous heating drives a mineralogic transformation in mineralogical (hydrous salt) control to the intensity of the in- the hydrous salts that makes the formerly "dry" salt bed become teraction and the depth of thermal influence of recrystallisation "wet" sediment. and mobilisation textures. When a dyke-swarm intersects halite or anhydrite the thermally-driven recrystallisation and fluid mi- Before our work in the Fulda region (Schofield et al., 2014), the gration halo is more limited, as outlined in Figure 1 and Figure nature of igneous interactions with evaporites was understood 5a, d. to be mainly that documented by studies in areas with intrusives interacting with thick anhydrous halite and anhydrite beds. The In contrast, when a dyke swarm intersects an interval contain- heating haloes were seen as driving recrystallisation and brine ing hydrous salts such kainite, carnallite or gypsum, the heat- migration over limited lateral distances of a few metres. How- ing drives the expulsion of the bound-water at decomposition ever, the potash seam interactions in the Fulda region show this temperatures much lower than the salts melting point (Table 1). alteration distance can be much greater (hundreds of metres) id Such hydrous-salt intervals devolatise, fluidise and flow, with hydrous salt layers are heated. the effects of the heating halo extending much further away from the heat source, driven in part by steam-driven hydrofracturing. The surface geology in the Dallol Mound region of Ethiopia On cooling, the resulting mineralogy in the highly-deformed bed shows an even more impressive set of igneous dyke hydrat- is dominated by the anhydrous form of the devolatised salt, as in ed salt interactions (Warren, 2016). There the potash interval the sylvite unit after carnallite as seen in potash seams adjacent known as the Houston Formation is a tens-of-metres thick sec- to dykes in the Fulda Region (Figures 5b, c). tion of hydrated salts below the upper halite unit and atop the lower halite. When the rising igneous dyke swarm rose to the Closer in to the heat source, the basalt that has moved in along level of Houston Formation, it drove a broad linear devolatisa- the hydrous potash beds show abundant peperite textures (Fig- tion zone in the dyke-heated alteration halo. This, in turn, forced ure 5c; Schofield et al., 2014). Actually, this is a unique form of the formation of the closed anticlinal uplift structure that is the peperite that is tied to beds of hydrous evaporite. It forms outside Dallol mound. The release of MgCl2 during volatisation also ex- the usual scenario envisaged for peperite whereby molten igne- plains phreatic breakout features that are outlined by at-surface ous material interacts with wet sediment, with the water in the collapse dolines with their hot (104-108°C) brine lakes and un- wet sediment held in interparticle pores. usual bischofite (MgCl2) precipitates. Likewise, the same set of The classic definition of a peperite is that it is a "genetic term processes explains the occurrences of metres to tens of metres applied to a rock formed essentially in situ by disintegration of thick bischofite intervals that are intersected in cores in some magma intruding and mingling with unconsolidated or poorly of the potash exploration wells in the vicinity of Dallol Mound consolidated, typically wet sediments. The term also refers to (pers. obs). These are likely cavity fill deposits formed as a by- similar mixtures generated by the same processes operating at product of kainite and carnallite devolatisation sourced at the the contacts of lavas and other hot volcaniclastic deposits with level of Houston Formation. such sediments" (Skilling et al. 2002). This set of more mobile brine fluid escape features has implica- In the case of the bedded hydrous salt intervals, before the intru- tions for nuclear waste storage in halite successions where a stor- sion of the igneous heat source, there was little to no free water, age cavity may be in proximity to an interval of hydrous evapo- other than occasional brine inclusions in associated halite chev- rite salts. Halite-hosted purpose-built caverns in thick evaporite Page 7 www.saltworkconsultants.com
intervals are one of the safest places in the world to store waste review of magma–sediment mingling: Journal of Volcanology but perhaps not in parts of the salt succession that entrain beds of and Geothermal Research, v. 114, p. 1-17. hydrous salts such as carnallite or kainite (Warren, 2017). Steinmann, M., P. Stille, W. Bernotat, and B. Knipping, 1999, The corrosion of basaltic dykes in evaporites: Ar-Sr-Nd isotope References and rare earth elements evidence: Chemical Geology, v. 153, p. Bastow, I. D., A. D. Booth, G. Corti, D. Keir, C. Magee, C. A.-L. 259-279. Jackson, J. Warren, J. Wilkinson, and M. Lascialfari, 2018, The development of late-stage continental breakup: Seismic reflec- Strauch, B., M. Zimmer, A. Zirkler, S. Höntzsch, and A. M. tion and borehole evidence from the Danakil Depression, Ethio- Schleicher, 2018, The influence of gas and humidity on the min- pia: Tectonics, v. 37. eralogy of various salt compositions – implications for natural and technical caverns: Advances in Geoscience, v. 45, p. 227- Beer, W., and L. Barnasch, in press, Werra-Fulda-Becken, 233. SDGG- Monography. Wall, M., J. Cartwright, R. Davies, and A. McGrandle, 2010, Chambefort, I., J. H. Dilles, and A. J. R. Kent, 2008, Anhy- 3D seismic imaging of a Tertiary Dyke Swarm in the Southern drite-bearing andesite and dacite as a source for sulfur in mag- North Sea, UK: Basin Research, v. 22, p. 181-194. matic-hydrothermal mineral deposits: Geology, v. 36, p. 719- 722. Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3- 319-13511-3): Berlin, Springer, 1854 p. Grishina, S., J. Dubessy, A. Kontorovich, and J. Pironon, 1992, Inclusions in salt beds resulting from thermal metamorphism by Warren, J. K., 2017, Salt usually seals, but sometimes leaks: Im- dolerite sills (eastern Siberia, Russia): European Journal of Min- plications for mine and cavern stabilities in the short and long eralogy, v. 4, p. 1187-1202. term: Earth-Science Reviews, v. 165, p. 302-341. Grishina, S., J. Pironon, M. Mazurov, S. Goryainov, A. Pustil- Warren, J. K., A. Cheung, and I. Cartwright, 2011, Organic nikov, G. Fonderflaas, and A. Guerci, 1998, Organic inclusions Geochemical, Isotopic and Seismic Indicators of Fluid Flow in in salt - Part 3 - Oil and gas inclusions in Cambrian evaporite Pressurized Growth Anticlines and Mud Volcanoes in Modern deposits from east Siberia - A contribution to the understanding Deepwater Slope and Rise Sediments of Offshore Brunei Darus- of nitrogen generation in evaporite: Organic Geochemistry, v. salam; Implications for hydrocarbon exploration in other mud 28, p. 297-310. and salt diapir provinces (Chapter 10), in L. J. Wood, ed., Shale Tectonics, v. 93: Tulsa OK, AAPG Memoir 93 (Proceedings of Gutsche, A., 1988, Mineralreaktionen und Stotransporte an ei- Hedberg Conference), p. 163-196. nem Kontakt Basalt-Hartsalz in der Werra-Folge des Werkes Hattorf: Unpubl. diploma thesis, thesis, Georg-August-Univer- Zimbelman, D. R., R. O. Rye, and G. N. Breit, 2005, Origin of sita, Gottingen. secondary sulfate minerals on active andesitic stratovolcanoes: Chemical Geology, v. 215, p. 37-60. Humphris, S. E., P. M. Herzig, D. J. Miller, J. C. Alt, K. Becker, D. Brown, G. Brugmann, H. Chiba, Y. Fouquet, J. B. Gemmell, G. G., M. D. Hannington, N. G. Holm, J. J. Honnorez, G. J. Iturrino, R. Knott, R. Ludwig, K. Nakamura, S. Petersen, A. L. Reysenbach, P. A. Rona, S. Smith, A. A. Sturz, M. K. Tivey, and X. Zhao, 1995, The internal structure of an active sea-floor mas- sive sulphide deposit: Nature, v. 377, p. 713-716. Knipping, B., 1989, Basalt intrusions in evaporites: Lecture Notes in Earth Sciences (Springer-Verlag), v. 24, p. 132 pp. Knipping, B., and A. G. Hermann, 1985, Mineralreaktionen und Stoff transporte an einem Kontakt Basalt-Carnallitit im Kalisalz- horizont Thüringen der Werra-Serie des Zechsteins: Kali und Steinsalz, v. 9, p. 111-124. Luhr, J. F., 2008, Primary igneous anhydrite: Progress since its recognition in the 1982 El ChichÛn trachyandesite: Journal of Volcanology and Geothermal Research, v. 175, p. 394-407. Schofield, N., I. Alsop, J. Warren, J. R. Underhill, R. Lehné, W. Beer, and V. Lukas, 2014, Mobilizing salt: Magma-salt interac- tions: Geology, v. 42, p. 599-602. Skilling, I. P., J. D. L. White, and J. McPhie, 2002, Peperite: a
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